In general, when a nut is tightened onto a bolt, a counteracting torque is applied to the head portion of the bolt in order to prevent the bolt from rotating during the tightening operation. When the head portion of the bolt is not accessible to a worker's hand or tool, the reacting torque must be applied to the bolt in some other suitable way.
As an example of a method used in automotive vehicle assembly lines a resisting plate of a size large enough to abut surrounding a vehicle parts is attached to the bolt, thus restricting rotation thereof due to contact of the resisting plate with other vehicle parts and then the nut is tightened onto the bolt.
This method is considered to be uneconomical as it requires an extra operation to clamp the resisting plate onto the bolt head and wastes material. It is also well-known to tighten the nut onto the bolt by restricting the rotation of the bolt with a mechanical wrench. A bolt capable of being tightened by a mechanical wrench has a pin-tail section adapted to be gripped by an inner socket of the wrench so that torque may be applied in the direction opposite to the direction of tightening the nut. Outer socket of the wrench grips the nut to rotate the latter in the tightening direction to tighten the nut at a predetermined torque. To attach the nut to the bolt prior to the tightening, the nut is preliminarily attached to the bolt which is inserted in its predetermined position. In the prior art, the pin-tail portion is coaxial with the threaded portion of the bolt. Therefore, in order for the pin-tail portion to protrude sufficiently far from the nut to allow the pin-tail portion to be successfully gripped by the wrench, the nut has to be rotated several turns. Therefore, the working efficiency of the pre-attaching operation was quite low, leading to a low efficiency of the entire assembly line.
On the other hand, in automotive vehicle assembly lines and the like, it is necessary to tighten a relatively large number of nuts to bolts within a relatively short period of time. Therefore, the period of time allowed for the preliminary attachment of each nut to its bolt is extremely short. At the same time, however, it is necessary to allow the pin-tail section to protrude sufficiently far from the nut to enable the inner socket of the wrench to grip it. As a result, in an automated line, such as an automotive vehicle assembly line, wherein pre-attaching and tightening are performed at different stations, this method has not been considered to be practical due to low efficiency.
In tightening the nut to the bolt, the required tightening torque varies according to the part of the vehicle to which the bolt is applied. Therefore, when several nuts must be tightened at different required torques, the tightening torques between vehicles tend to be nonuniform. Accordingly, in order to obtain a uniform torque, the worker on the line has to perform the tightening operation very carefully. This results in lower working efficiency. However, even when tightening operation is performed, very carefully, some nonuniformity among tightening torques is inevitable. However, in automotive vehicle manufacture, maintaining the predetermined tightening torque is very important for the security of the vehicle. Insufficient tightening torque may seriously influence the operating safety the vehicle.
With the foregoing problems in the prior art in mind, the present invention is intended to provide a bolt which make tightening of the nut with a mechanical wrench easier in order to improve efficiency and productivity in an automated manufacturing line.
Another and more specific object of the present invention is to provide a bolt which exposes a sufficiently long pin-tail during the step of pre-attaching the nut to the bolt for tightening with a mechanical wrench.